In the field of wireless communication systems, it is known generally that power may be conserved by switching off components when they are not in use. Power conservation is known to be particularly desirable in battery powered apparatus.
One power hungry component is the power amplifier in a transmitter. However, if switched off to conserve power, when switched on again the power amplifier is unstable for a time, thus producing a power transient.
Compensation (or tracking) schemes are known for accommodating such a power transient. Such compensation schemes may be implemented in the transmitter or in a receiver.
For single-carrier systems such power amplifier transient tracking in the receiver in effect forms part of the automatic gain control of the system. One example of this is disclosed in G. Travares and M. S. Piedade, “High performance algorithms for digital signal processing AGC”, IEEE International Symposium on Circuits and Systems, vol 2, p 1529-1532, 1990.
However, in multi-carrier systems, e.g. OFDM systems such as HIPERLAN/2 (as specified by the ETSI normalisation committee, broadband Radio Access networks (BRAN), HIPERLAN Type 2), the large peak-to-average ratio of the received signal precludes the use of the automatic gain control to compensate for a power amplifier transient, and thus automatic gain control must be performed only in the preamble of the frame, for example as disclosed by WO-0079748.
For this reason many transmit-side compensation schemes have been proposed, for example as disclosed by U.S. Pat. No. 527,415. However, these schemes are complex to implement.
Certain other tracking schemes, for example as disclosed by M. Sandell and O. Edfors, “A comparitive study of pilot-based channel estimators for wireless OFDM”, Research Report TULEA 1996, Lulea University of Technology, track pilots in multi-carrier systems, but this is for the purpose of correcting for time-varying channels. Moreover, such schemes rely on additional pilots that are not present in many OFDM systems, including HIPERLAN/2.
XP-002212283 discloses a multi-carrier radio communication unit, where large fluctuations in transient interference due noise is prevented in a variable gain amplifier. Switching an AGC circuit into the signal path, or using a control voltage when a low-level signal is received prevents the transient interference. A need therefore exists for a method of compensating a power amplifier transient in OFDM systems, wherein the above mentioned disadvantages may be alleviated.